A gas sensor is a subclass of chemical sensors that measures the concentration of gases in the vicinity of the sensor. A gas interacts with the sensor and provides a measure of the concentration of the gas based on a signal or property change created by the interaction. Gas sensors can include sensors based on measuring changes in electrical properties of the sensor, such as, but not limited to, metal oxide semiconductor sensors, polymer coating based sensors, carbon nanotube based sensors, and moisture adsorbing material based sensors. Other types of gas sensors can include sensors based on measuring other properties, such as, but not limited to, optical, acoustic, gravimetric, gas chromatograph, flame ionization, and calorimetric based sensors.
Generally, gas sensors are designed to detect a specific type of gas or class of gases. For example, there are specific gas sensors designed to have sensitivity to specific gases such as carbon monoxide, carbon dioxide, nitrous oxide compounds, ammonia, methane, sulfur dioxide, hydrogen sulfide, methanol, ethanol, and volatile organic compounds. The signal generated by the interaction of gases with a sensor is related to the total concentration of all types of gases that the sensor can detect. In other words, a gas sensor may not be discriminatory with respect to the different gases that the sensor is able to detect at selected operating conditions since the total signal generated is a function of the responses from all detected gases. Therefore, a common approach for gas sensors is to design a sensor to detect a specific type of gas while minimizing sensitivity to other types of gases, or to provide a means to separate gas species and measure one at a time.
For applications in mobile devices and consumer devices where the context varies greatly, it can be desirable for a gas sensor system to provide resolution sufficient to characterize or classify a gas or mixture of gases in order to determine the context, as a human does via its sense of smell. The value of an electronic nose for mobile and consumer applications is not limited to an analytical measurement of a particular gas concentration. Rather, it can be desirable to capture the signature or characteristics associated with a mixture of volatile organic for context identification or comparison. Various uses might include, but are not limited to, identification of a smell, an environment, or a combination of chemicals triggering another measurement or a report, validation of a material's authenticity or capturing a snapshot of particular smell or environment for data aggregation. In such applications, analytical grade measurements may not be necessary. However, the more characteristics of a gas mixture that can be measured, the better. For consumer and mobile applications, cost, size, speed of measurement and power consumption are also important. Common lab analytical equipment often does not meet desired specifications or budget. There is a need in the art for a compact, low cost, low power, rapid gas characterization system with resolution improved over individual or even simple arrays gas sensors. Improved sensitivity to trace concentrations may also be desirable.
An array of gas sensors with varied selective responses can provide a measure of chemical selectivity. For instance, for a given volatile organic compound (VOC), a metal oxide (MOX) gas sensor whose resistance varies with the concentration of combustible gas, has a particular temperature at which the response is maximum for a given gas. In theory, a temperature scan of a MOX sensor would identify a molecular species. However, many chemicals can have the same or similar temperature-dependent responsivities, so this kind of selectivity can have practical limits, especially where the sensor context (location, environment, etc.) varies widely.
In another example of an array, MOX sensors can be provided with different metal oxide materials. The different materials provide different responses to gases. However, yet again, many analytes or combinations of analytes may have similar and undifferentiated responses. Given an array with limited selectivity or resolution, another axis of resolution or selectivity is desired for characterization or classification of gas mixtures.